Conventional vertical axis wind turbines (VAWTs) are wind turbines that comprise multiple rotor blades that can rotate, when impinged upon by wind, about a vertical axial rotor shaft. One configuration that VAWTs can take is the conventional “egg-beater” Darrieus-type VAWT described in U.S. Pat. No. 1,835,018. VAWTs have several advantages over horizontal axis wind turbines. For example, VAWTs do not need to be pointed into the wind. Thus, they do not require complex yaw control mechanisms. Further, generators and gearboxes can be located close to the ground. This allows, for example, easy access to these components for maintenance and eliminates the need for a large support tower to hold these components off the ground.
However, VAWTs have susceptibilities of their own to overcome. VAWTs are typically supported by a set of guy-wires that hold the turbine in its vertical orientation and stabilize the VAWT against, for example, large gusts of wind and vibrations that occur as the rotor blades rotate. The guy-wires typically extend directly from the top of the vertical axial rotor shaft to the ground and exert compressive forces on the central tower supporting the turbine assembly. These forces are borne by the lower bearing assembly, which is already carrying the weight of the VAWT. This additional load can reduce the lifetime of the lower bearing assembly.
In addition, VAWTs are susceptible to dynamic stall. FIG. 1 shows a schematic transverse section of a VAWT with a rotor blade 110 located at various possible azimuthal angles 111-118 (θ=0°, 45°, 90°, 135°, 180°, 225°, 270°, and 315° respectively) about a vertical rotational axis 120. Eight blades are shown to illustrate eight respective azimuthal angles. In many embodiments, the transverse section at a given instant in time would reveal any two blades on opposing respective sides of the vertical axis 120, as will be described below. As the rotor blade 110 rotates clockwise about the vertical axis 120, the rotor blade 110 experiences varying angles of attack α relative to incident wind. The angle of attack α is the angle between the oncoming wind and the chord of the rotor blade. The oncoming wind vector is the vector sum of the incident wind velocity vector and the velocity of a rotating rotor blade. At low angles of attack, air flows smoothly over the surfaces of the rotor blade 110 and the blade experiences lift, which is useful for urging continued rotation of the blade 110 about the vertical axis 120. This lift increases with increasing angle of attack up to an angle at which flow separation begins at the rotor blade. When the flow of air begins to separate from a blade surface, lift no longer increases; in fact, lift may drop suddenly while large eddy currents are generated in the blade's wake. Thus, there is a critical angle of attack at which blade experiences maximum lift. As the angle of attack α continues to increase, the flow of air in the blade's wake becomes increasingly turbulent. At attack angles beyond the critical angle, the lift and pitching moments experienced by the blade 110 decrease sharply and are accompanied by a large increase in drag, as the rotor blade stalls. The ability of a VAWT to generate power is reduced whenever one or more rotor blades experience stall conditions, and rapid changes in the pitching moment can be destructive to the VAWT. Thus, it is desirable that stall conditions be avoided, or at least minimized.
VAWT stall conditions experienced by rotor blades are dynamic in that the blades can transition in and out of regions where stall conditions are experienced as the VAWT rotates about its vertical rotational axis 120. The regions where rotor blades experience stall conditions as it rotates about the vertical rotational axis 120 are referred to as “dynamic stall regions.” Rotor blade 110 experiences dynamic stall regions 130, 140. The rotor blade 110 does not have to transition in and out of the dynamic stall regions 130, 140 at any specific azimuthal angles suggested by FIG. 1. Rather, FIG. 1 is meant to show that VAWT rotor blades experience stall conditions at the highest angles of attack, or, when the rotor blade 110 is at azimuthal angles in respective regions about θ=90 degrees and θ=270 degrees (FIG. 1). In this specification, the terms “stall” and “dynamic stall” are used interchangeably. In a conventional Darrieus VAWT design, dynamic stall can start near the top of a vertical axial rotor shaft (vertical support column) where the tip speed ratio (i.e., the ratio of the rotational speed of the rotor blades to the wind speed) is lowest. Thus, the upper portion of Darrieus VAWTs can operate in dynamic stall conditions for a large portion of time during each revolution.
One way of reducing dynamic stall is to reduce the angular width of the dynamic stall regions. To such end, the effects of various active flow control techniques on boundary layer separation have been studied. “Active flow control” refers to the injection or removal of air to the flow of air over an airfoil surface. For example, the application of periodic excitation (alternating blowing and suction) as an active control of separation technique on NACA 0015 airfoils has been studied. D. Greenblatt et al., “Dynamic Stall Control by Periodic Excitation, Part 1: NACA0015 Parametric Study” Journal of Aircraft, Vol. 38, pp. 430-439, 2001. In addition, flight tests have been performed to assess the effectiveness of using electromagnetic actuators for active flow control in tiltrotor aircraft. A. McVeigh et al., “Model and Full Scale flight Tests of Active Flow Control on a Tilt Rotor Aircraft,” (presented at the American Helicopter Society 60th Annual Forum, Baltimore, Md., Jun. 7-10, 2004). However, these active flow control approaches involve the use of actuators that can be complex, heavy, and hard to maintain because of multiple moving parts and that require the consumption of power to operate. Thus, these approaches can be costly, in terms of both initial manufacturing and on-going maintenance expenses.